1. Field of the Invention
The present invention relates to a color wheel suitable for use as a filter element of a time-share light dispersing device, and to a color wheel incorporated in a color wheel assembly making up a projection-type image display apparatus.
2. Description of the Related Art
Color composition in a projection-type image display apparatus has conventionally been accomplished commonly by a method, such as: a single-panel method, in which one light valve element adapted to control light amount per pixel thereby creating an image is used to disperse each pixel into red (R), green (G), and blue (B) lights; and a three-panel method, in which three light valve elements dedicated to R, G and B lights, respectively, are used to produce in parallel R, G and B images, and then the three images thus produced are composed. Recently, as a light valve element capable of fast switching, such as a ferroelectric liquid crystal display element or a digital micro mirror device, is increasingly coming into practical use, a time-sharing single-panel method is widely used. In the time-sharing single-panel method, R, G and B lights are caused to sequentially impinge on one light valve element, the light valve element is driven in synchronization with switching-over of the R, G and B lights thereby producing R, G and B images in a time series manner, and the images thus produced are projected onto a screen, or the like. Here, color composition of the images is accomplished by a viewer due to an afterimage effect occurring at a sense of vision. In the time-sharing single-panel method, reduction in both dimension and weight of the apparatus, which is a feature of a single-panel method, can be achieved by employing a relatively simple optical system, and therefore the time-sharing single-panel method is favorable for realizing inexpensive fabrication of a projection-type image display apparatus. In such an image display apparatus, a color wheel is preferably used as a filter element of a time-share light dispersing device to sequentially disperse light emitted from a white light source into R, G and B lights having respective wavelength bands in a time-sharing manner (refer to, for example, Japanese Patent Application Laid-Open No. H06-347639).
FIGS. 7A and 7B are respectively front and side views of a typical color wheel assembly incorporating such a color wheel. Referring to FIG. 7B, a color wheel assembly 200 comprises a color wheel 100, a hub 105, and a motor 106. The color wheel 100 is a tricolor color wheel structured such that a disk-like substrate 101, which is made of a light-transmitting material, for example, optical glass, has three filter sectors 102, 103 and 104 formed on one surface thereof, and such that, for example, the filter sector 102 transmits R light only, the filter sector 103 transmits G light only, and the filter sector 104 transmits B light only. The color wheel 100 thus structured is fixedly attached to the motor 106 via the hub 105 coaxially therewith. The color wheel assembly 200 operates such that the color wheel 100 is rotated by the motor 106 so that the filter sectors (R, G and B) 102, 103 and 104 sequentially have white light S falling incident thereon whereby the white light S is sequentially dispersed into R, G and B lights.
FIG. 8A is a plan view of the aforementioned color wheel 100, and FIG. 8B is a schematic cross-sectional view taken along a line A-A′ of FIG. 8A. The filter sectors 102, 103 and 104 are usually constituted by optical interference filters of dielectric multi-layer films structured such that a dielectric thin film formed of a material having a high refractive index (e.g., TiO2, ZrO2, and ZnS), and a dielectric thin film formed of a material having a low refractive index (e.g., SiO2, and MgF2) are alternately laminated by an evaporation method, a sputtering method, or the like. The optical interference filter is superior in durability (heat resistance, light stability, and chemical resistance) to a color filter formed by a staining method, a pigment dispersion method, or the like, has a high transmittance, and readily achieves a sharp spectroscopic characteristic, and therefore endures exposure to intensive light flux and produces a high display quality image.
Adjacent filter sectors are required to abut each other precisely and tightly unless achromatic areas which do not constitute any filter sectors are intentionally disposed. This is because if the adjacent filter sectors do not abut each other precisely and tightly, a gap is generated between the adjacent filter sectors, and light passing the gap fails to definitely determine its color thus resulting in not fully contributing to forming an image. When filters are formed by an evaporation method or a sputtering method, a metal mask formed of a metallic thin plate and having openings corresponding to the filter sectors is preferably used for demarcating the filter sectors. The metal mask is first guided mechanically, for example, with a positioning pin, and then finally lined up by viewing, for example, through a microscope, the peripheries of filter sectors of one kind already formed and the openings of the metal mask.
However, the following problem is found in the positioning technique described above. It occasionally happens at the process of forming the filter sectors due to the thickness of the metal mask that as shown in FIG. 8B, dielectric multi-layer films constituting the filter sectors 102, 103 and 104 (103 not shown in FIG. 8B) fail to achieve a predetermined thickness at regions F which extend along the outlines of the openings of the metal mask, and which measure up to about 100 μm in width. In such a case, it is difficult to clearly determine the demarcation of the filter sectors even by viewing through a microscope, and this hinders precise alignment of the openings to the filter sectors. Consequently, the filter sectors thus formed are positioned with respect to one another with a lowered degree of accuracy, and an incomplete filter portion E is inevitably found, for example, between the filter sectors 102 and the filter sectors 104 as shown in FIG. 8B. Referring to FIG. 9, out of light rays A to D passing the incomplete filter portion E, the light rays A and D may possibly contribute to forming an image but the light rays B and C definitely fail to do so.
In order to overcome the problem, for example, Japanese Patent Application Laid-Open No. H11-222664 discloses a metal mask with openings, in which the sidewalls of the openings are inclined with respect to the metal mask surfaces such that the openings have an increased area at one of the surfaces facing an evaporation source so that particles from the evaporation source come into the openings with reduced restriction thereby better achieving uniform film formation within the openings.
The aforementioned Japanese Patent Application Laid-Open No. H06-347639 discloses that filter sectors are desired to abut each other unless achromatic areas which do not constitute any filter sectors are intentionally disposed, but does not teach how it can be achieved. Also, the aforementioned Japanese Patent Application Laid-Open No. H11-222664 indicates a method that is anticipated to be good to a certain degree for clearly demarcating a boundary between filter sectors provided that an optimum inclination angle of the sidewalls surely exists and can be obtained somehow which allows a film to be formed uniform in thickness all the way up to the peripheries of filter sectors. The optimum inclination angle of the sidewalls, however, must be obtained theoretically and experimentally based on various considerations, such as a film material method and conditions of film formation, a desired film thickness, a metal mask thickness, and the like, and therefore the method disclosed therein cannot be readily applied to fabrication of a color wheel.